The present invention relates to a solid acid catalyst having a high activity in an acid-catalyzed reaction system and having easy handleability and a method for producing the same.
In the chemical industry, there have been known acid-catalyzed reactions such as alkylation reactions, esterification reactions, and isomerization reactions. Hitherto, in these reactions, there have been used acid catalysts such as sulfuric acid, aluminum chloride, hydrogen fluoride, phosphoric acid, and paratoluenesulfonic acid. However, these acid catalysts have metal corroding properties so that the use of costly anticorrosive materials for a production apparatus or anti-corrosive treatment of a production apparatus has been required. Usually, not only is it difficult to separate the catalysts from the reactants after the reaction but also it is necessary to conduct disposal of waste acid and it is also inevitable to undergo a complex process such as alkali washing. Also, there are many problems from the environment viewpoints. Further, it has been very difficult to reuse the catalysts.
In order to solve these problems, there has been proposed a solid acid catalyst containing a sulfate group which has been obtained by contacting a hydroxide or hydrated hydroxide of a metal belonging to group IV of the Periodic Table with a solution containing a sulfureous component and calcining the mixture at 350 to 800xc2x0 C. (Japanese Patent Publication No. 59-6181). The solid acid catalyst has an acidity higher than that of 100% sulfuric acid (Hammett acidity function H0 is xe2x88x9211.93). Because of their high acidity, the solid acid catalysts exhibit high catalyzing power in various acid-catalyzed reactions and have advantageous features that they show low corrosiveness, can be separated easily from the reactants, do not require disposal of waste acids, and can be reused, so they are expected to be substituted for conventional acid catalysts.
It has been also known that a catalyst obtained by impregnating platinum into a catalyst that has been obtained by the calcination of a sulfureous component-containing zirconia gel exhibits good activity for an isomerization reaction of hydrocarbons (U.S. Pat. No. 3,032,599).
As the methods for producing metal oxide catalysts that contain a platinum-family metal and a sulfureous component and are used mainly in the isomerization of hydrocarbons, there have been disclosed a method in which the step of calcination has been eliminated between the steps of treatment with a sulfureous compound and supporting of a platinum-family metal, a method in which the order of the steps of treatment with a sulfureous compound and supporting of a platinum-family metal are reversed, and a method in which the kind of sulfureous compound is changed in Japanese Patent Publication Nos. 5-29503, 5-29504, 5-29505, and 5-29506.
Also, it has been known that a solid acid catalyst obtained by adding a sulfureous compound to aluminum hydroxide or oxide followed by calcination exhibits an acidity higher than 100% sulfuric acid (Japanese Patent Application Laid-open No. 5-96171, Arata, Trends in Physical Chemistry, vol. 2, item 1 (1991)).
Japanese Patent Application Laid-open No. 9-38494 discloses a method for the production of a metal oxide shaped catalyst treated with a sulfate group. The method is characterized by preliminarily calcining a shaped material shaped from a metal hydroxide and boehmite at a temperature of 300xc2x0 C. to 500xc2x0 C. and then treating it with a sulfate group, which is a method for shaping a catalyst utilizing alumina as a binder. However, its catalytic activity is decreased as compared with a powder catalyst containing no boehmite due to the shaping with the addition of the boehmite. Also, it is disclosed that a shaped material obtained by using a metal hydroxide and boehmite powder, when dried at a temperature below 300xc2x0 C., will be pulverized and destroyed by addition of water and that a catalyst obtained by kneading a mixture of platinum-containing sulfated zirconia catalyst powder (a powdery catalyst composed of zirconia supporting thereon platinum and a sulfate group) and boehmite powder with the addition of water, followed by shaping and calcination has a greatly decreased catalytic activity.
Solid catalysts, when utilized, must be shaped catalysts, which are easy to separate from the reactants and reuse as opposed to the powder form. However, even if catalysts have sufficient catalytic activity when they are in the form of a powder, shaping of them will make it impossible to obtain the mechanical strength that is required at the time of reaction/production or their catalytic activity will decrease accompanying their shaping. Thus, there has been made no report on a method for producing a solid acid shaped catalyst that satisfies the characteristics required as a catalyst and has a required mechanical strength.
The present invention solves these problems and provides a solid acid catalyst having a sufficiently high activity, an easy handleability sufficient for practical use and a sufficient mechanical strength, and which is a shaped product of the solid acid catalyst containing a sulfureous component, a method for producing the same, and a reaction method using such a catalyst.
As a result of extensive research on a method for producing the solid acid catalyst, the present inventors have now found that kneading zirconium hydroxide having specific physical properties, pseudoboehmite having specific physical properties, and ammonium sulfate, followed by shaping and calcination can give rise to a solid acid catalyst having an excellent catalytic activity and a sufficient mechanical strength, and further made research on the catalyst to complete the present invention.
According to a first aspect of the present invention, a method for producing a solid acid catalyst comprises the steps of: fabricating a support comprising a portion of zirconia and/or hydrated zirconia and a portion of alumina and/or hydrated alumina and having a peak diameter in the range of 0.05 to 1 xcexcm in a pore diameter distribution of 0.05 to 10 xcexcm; and having a sulfureous component supported on the support.
According to a second aspect of the present invention, a method for producing a solid acid catalyst comprises the steps of: fabricating a support comprising a portion of zirconia and/or hydrated zirconia and a portion of alumina and/or hydrated alumina in which pores having a pore diameter of not less than 0.05 xcexcm and not more than 1 xcexcm occupy a pore volume of 0.05 to 0.5 ml/g and pores having a pore diameter of above 1 xcexcm and not more than 10 xcexcm occupy a pore volume of below 0.05 ml/g; and having a sulfureous component supported on the support.
According to a third aspect of the present invention, a method for producing a solid acid catalyst comprises the steps of: kneading powder comprising zirconium hydroxide and/or hydrated oxide whose agglomerated particles have an average particle diameter of 0.2 to 10 xcexcm with powder comprising aluminum hydroxide and/or hydrated oxide having a fibrous particle form, shaping the mixture to fabricate a support; and having a sulfureous component supported on the support. In this case, it is preferred that the above-mentioned support has a peak diameter in the range of 0.05 to 1 xcexcm in a pore diameter distribution of 0.05 to 10 xcexcm and in which pores having a pore diameter of not less than 0.05 xcexcm and not more than 1 xcexcm occupy a pore volume of 0.05 to 0.5 ml/g and pores having a pore diameter of above 1 xcexcm and not more than 10 xcexcm occupy a pore volume of below 0.05 ml/g. It should be noted that it is preferred to conduct the step of fabricating a support and the step of having a sulfureous component supported on the support in one step simultaneously.
The solid acid catalyst of the present invention is a catalyst which is constituted by a portion of zirconia and/or hydrated zirconia and a portion of alumina and/or hydrated alumina, contains a sulfureous component, and is used in acid-catalyzed reactions, the catalyst having a pore diameter distribution of 0.05 to 10 xcexcm with a peak pore diameter being in the range of 0.05 to 1 xcexcm. Alternatively, it is a catalyst which is constituted by a portion of zirconia and/or hydrated zirconia and a portion of alumina and/or hydrated alumina, contains a sulfureous component and is used in acid-catalyzed reactions, in which catalyst pores having a pore diameter of not less than 0.05 xcexcm and not more than 1 xcexcm occupy a pore volume of 0.05 to 0.5 ml/g and pores having a pore diameter of above 1 xcexcm and not more than 10 xcexcm occupy a pore volume of below 0.05 ml/g. For the acid-catalyzed reactions, the above-mentioned solid acid catalyst is used preferably for the conversion reaction of hydrocarbons.
Zirconia Powder
The powder comprising zirconium hydroxide and/or hydrated oxide for use in the production of solid acid catalysts (hereinafter, simply referred to as xe2x80x9czirconia powderxe2x80x9d) has an increased crushing strength, and zirconia tends to be stabilized when the zirconia powder is converted into an amorphous form, which has no definite crystal structure as examined by X-ray or electron beam diffraction analysis. Use of agglomerated particles having an average particle diameter of 0.2 to 10 xcexcm, particularly 0.2 to 5 xcexcm, and more particularly 0.5 to 2 xcexcm is preferred for increasing the activity and mechanical strength of the catalyst. The average particle diameter of agglomerated particles can be measured, for example, by a method which involves irradiating a laser beam onto a group of particles dispersed in water and calculating from the scattered light.
The zirconia powder may be produced by any method but generally it can be obtained by the neutralization or hydrolysis of zirconium salts or organic metal compounds, for example, oxychlorides, alcoholates, chlorides, sulfates, nitrates, and oxysulfates. The main component of zirconia powder is a mixture of zirconium hydroxide and hydrated zirconium oxide, zirconium hydroxide, or hydrated zirconium oxide.
Further, the zirconia powder can be used as a complex metal hydroxide and/or hydrated complex metal oxide. To the hydroxide and/or hydrated oxide of zirconium may be added hydroxides and/or hydrated oxides of other metals. As the other metals there can be advantageously used titanium, hafnium, vanadium, chromium, manganese, iron, silicon, tin, gallium, etc. Compounds of such other metals may be complex metal compounds. However, as the zirconia powder, there are used preferably those which consist substantially of only zirconium as the metal component, more specifically, those which contain zirconia as a metal in an amount of at least 70% by weight, particularly at least 90% by weight, based on the total weight of metals in the zirconia powder.
Alumina Powder
The powder which comprises aluminum hydroxide and/or hydrated oxide for use in the production of the solid acid catalyst (hereinafter, simply referred to as xe2x80x9calumina powderxe2x80x9d) preferably has a fibrous particle form in order to increase the mechanical strength of the shaped catalyst, particularly the water stability of the shaped pellets. More specifically, the shape of the fibrous particle form is preferably such that its aspect ratio is greater than 10, particularly greater than 20. Usually, the upper limit of the aspect ratio is about 200. Here, the aspect ratio means a ratio of the lengths of the minor axis and the major axis of a particle ([length of major axis]/[length of minor axis]) and can be obtained, for example, by observing the alumina powder on a transmission electron microscope or the like, measuring the ratios of the lengths of the minor axis and the major axis, and calculating their average values. When the particles are spherical, the aspect ratio is 1, which is the least value. Typically, such a particle form can be obtained as a primary particle of 0.1 xcexcm or more in the major axis and a secondary particle whose primary particles are oriented in a certain direction. Also, particles of other forms than fibrous form, for example, plate-like particles, may be contained so far as the aspect ratio that is an average value falls within the range of greater than 10, particularly greater than 20.
The alumina powder is preferably of such a form that its agglomerated particles have an average particle diameter of 0.5 to 50 xcexcm, particularly 1 to 40 xcexcm, and more particularly 1 to 20 xcexcm. Usually, the agglomerated particle is an agglomerate of fibrous particles. As the alumina powder, there can be used those manufactured by various production methods. Particularly, it is preferred to use aluminum hydrated oxide having a boehmite structure, such as pseudoboehmite, because there can be obtained an increased catalytic activity. Use of xcex1-alumina or xcex3-alumina as the alumina powder will result in a decrease in mechanical strength relatively and a decrease in catalytic activity.
Support
The support which can preferably be used in the present invention has a peak diameter in the range of 0.05 to 1 xcexcm in a pore diameter distribution of 0.05 to 10 xcexcm, and particularly its pores having a pore diameter of not less than 0.05 xcexcm and not more than 1 xcexcm occupy a pore volume of 0.05 to 0.5 ml/g and pores having a pore diameter of above 1 xcexcm and not less than 10 xcexcm occupy a pore volume of below 0.05 ml/g. It is preferred for increasing the mechanical strength of the catalyst that it has a peak diameter in the range of 0.05 to 1 xcexcm, particularly 0.05 to 0.5 xcexcm, but no other peak in a pore diameter distribution of 0.05 to 10 xcexcm and that pores having a pore diameter of not less than 0.05 xcexcm and not more than 1 xcexcm occupy a pore volume of 0.05 to 0.5 ml/g, particularly 0.05 to 0.3 ml/g, and pores having a pore diameter of above 1 xcexcm and not more than 10 xcexcm occupy a pore volume of below 0.05 ml/g, particularly below 0.02 ml/g.
The pore distribution can be measured by a mercury injection method in which it is assumed that the contact angle of mercury is 140xc2x0 and surface tension is 480 dynes/cm and all the pores are cylindrical. The pore size distribution having a peak means that a so-called pore distribution curve obtained by plotting a differential value of cumulative pore volume by pore diameter versus pore diameter has a clear optimal value.
The support is not a powder but of a shaped form and it is easy to obtain a support of 0.5 to 20 mm in size. Usually, particles having a size (length of cross-section) of 0.2 to 50 mm, particularly 0.5 to 20 mm, are used preferably. The alumina portion and zirconia portion are present in the support as particles of 0.01 to 100 xcexcm. Such a support can be fabricated by kneading the above-mentioned zirconia powder and alumina powder with each other and shaping the mixture. However, those supports fabricated by other methods may be used so far as they have the predetermined pore structure.
For the kneading, there may be used a kneader used generally in the preparation of catalysts. Usually, a method in which raw materials are charged in the kneader, water is added thereto, and mixed with stirring vanes can be used advantageously. However, no particular limitation is posed on the order of addition of raw materials and water. When kneading, usually water is added. However, it is not always necessary to add water when the raw material powders are in the form of slurry. The liquid to be added may be an organic solvent such as ethanol, isopropanol, acetone, methyl ethyl ketone, and methyl isobutyl ketone. The temperature and period of time of kneading may vary depending on the zirconia powder and alumina powder used as raw materials. However, there is no particular limitation on such conditions so far as the conditions can give rise to a preferable pore structure. Similarly, within the range in which the properties of the catalyst of the present invention are maintained, the kneading may be performed with the addition of an acid such as nitric acid, a base such as ammonia, an organic compound, a binder, a ceramic fiber, a surfactant, zeolite or the like.
The shaping after the kneading may be performed using a shaping method generally used in the preparation of a catalyst. Particularly, since shaping into any desired form, such as pellet form and honeycomb form, can be efficiently performed, extrusion molding using a screw type extruder can be employed preferably. The size of the shaped material is not limited particularly. However, usually, it is shaped so as to have a cross-section of 0.5 to 20 mm in length. For example, in the case of cylindrical pellets, there can be easily obtained those that usually have a diameter of about 0.5 to about 10 mm, and a length of about 0.5 to about 15 mm.
The calcination after the shaping is carried out in a gas atmosphere such as air or nitrogen. It is preferred that the calcination is performed also for the purpose of having a sulfureous component supported on the support since this makes the process simple.
Supporting of Sulfureous component
The sulfureous component can be supported by a support by contacting a sulfureous compound with the support followed by heat treatment. As the sulfureous compound, there can be cited, for example, sulfuric acid, ammonium sulfate, sulfurous acid, ammonium sulfite, and thionyl chloride. Ammonium sulfate and ammonium sulfite are preferred since they are less corrosive to the production apparatus. The sulfureous compound may be used as it is or as a solution such as an aqueous solution. The sulfureous compound may be solid or liquid and there is no particular limitation on the concentration of solutions so that it can be formulated taking into consideration the amount of solution necessary for kneading or the like. The amount of sulfureous compound to be added is preferably such that the amount of sulfur in the finally obtained solid acid catalyst occupies 0.2 to 10% by weight, particularly 1 to 10% by weight.
It is preferred to have the sulfureous compound supported on a support simultaneously with the fabrication of the support. The catalyst of the invention can be fabricated by kneading the zirconia powder, alumina powder, and sulfureous compound, shaping and calcining the resultant mixture. The kneading and shaping can be performed in the same manner as in the case of fabricating the support. It is preferred in view of catalytic activity that the weight of the sulfureous compound is 3 to 40% by weight, particularly 10 to 30% by weight, based on its total weight before calcination. The calcination is preferably carried out at a temperature at which a tetragonal crystal structure of zirconium oxide is obtained. This structure can be confirmed by X-ray diffraction using a CuKxcex1 ray, more particularly when an X-ray diffraction peak ratio of 2xcex8=28.2xc2x0 and 2xcex8=30.2xc2x0 (hereinafter, abbreviated as xe2x80x9cS28.2/S30.2 ratioxe2x80x9d; here, S28.2 designates the area of a peak of a tetragonal crystal of zirconia at 2xcex8=28.2xc2x0 while S30.2 designates the area of a peak of a tetragonal crystal of zirconia at 2xcex8=30.2xc2x0) is 1.0 or less, preferably 0.05 or less. The presence of substantially no monoclinic crystal structure results in a higher catalytic activity.
When pseudoboehmite type alumina is used as the alumina powder, a preferred temperature is 450 to 800xc2x0 C., particularly 500 to 800xc2x0 C., and more particularly 600 to 800xc2x0 C. and a preferred period of time of calcination is 0.1 to 20 hours. Too high a calcination temperature is undesirable since the proportion of monoclinic crystal in the crystal structure of zirconium oxide increases and the S28.2/S30.2 ratio may exceed 1 and results in a decrease in catalytic activity. Also, too low a calcination temperature is undesirable since zirconium oxide will not crystallize, with the result that catalytic activity will decrease.
In the case where a support is fabricated and then a sulfureous component is supported by the support, the sulfureous compound may be used in any form, for example, gas or an aqueous solution, so far as it can be brought into sufficient contact with the support. However, it is preferred that it is in the form of a liquid because of easier handleability. There is no particular limitation on the contacting method. However, an impregnation method by spraying, dipping, etc. and a method in which it is rendered gaseous and passed through a catalyst layer are used advantageously. After it is brought into contact with the sulfureous compound, the support is calcined at a temperature of higher than 300xc2x0 C. but lower than 800xc2x0 C., preferably higher than 400xc2x0 C. but lower than 800xc2x0 C. to obtain the target solid acid catalyst. The calcination time is usually 0.5 to 10 hours.
Used Catalyst
In the present invention, as the support there can be employed a used solid acid catalyst having a decreased activity. The solid acid catalyst before use comprises a support that is constituted by a zirconia portion having a tetragonal crystal structure and an alumina portion and a sulfureous component supported by the support. It is preferred that the support constituted by a zirconia portion having a tetragonal crystal structure and an alumina portion remains, even after use. Depending on the conditions under which it is used, there is the case where the support contains no sulfureous component.
The solid acid catalyst is preferably produced by kneading zirconia powder, alumina powder and a sulfureous compound, shaping and calcining. In this case, the kneading and shaping can be performed in the same manner as the fabrication of the support described above. In this case, it is preferred in view of catalytic activity that the weight of the sulfureous compound is 3 to 40% by weight, particularly 10 to 30% by weight, based on the total weight before the calcination. The calcination is performed at a temperature at which zirconium oxide having a tetragonal crystal structure can be obtained.
Solid Acid Catalyst
The solid acid catalyst of the present invention is a catalyst which comprises a support constituted by a portion of zirconia and/or hydrated zirconia (hereinafter, also referred to as xe2x80x9czirconia portionxe2x80x9d) and a portion of alumina and/or hydrated alumina (hereinafter referred to as xe2x80x9calumina portionxe2x80x9d) and a sulfureous component supported by the support, and which has a peak in diameter in the range of 0.05 to 1 xcexcm in a pore diameter distribution of 0.05 to 10 xcexcm, particularly pores having a pore diameter of not less than 0.05 xcexcm and not more than 1 xcexcm occupy a pore volume of 0.05 to 0.5 ml/g and pores having a pore diameter of above 1 xcexcm and not more than 10 xcexcm occupy a pore volume of below 0.05 ml/g. The pore structure of the catalyst can be measured in the same manner as the support and the pore structure with a pore diameter of 0.05 xcexcm or more is substantially the same before the supporting of the sulfureous component. In particular, it is preferred for increasing the mechanical strength of the catalyst that it has a peak diameter in the range of 0.05 to 1 xcexcm, particularly 0.05 to 0.5 xcexcm, but no other peak in a pore diameter distribution of 0.05 to 10 xcexcm, and that pores having a pore diameter of not less than 0.05 xcexcm and not more than 1 xcexcm occupy a pore volume of 0.05 to 0.5 ml/g and pores having a pore diameter of above 1 xcexcm and not more than 10 xcexcm occupy a pore volume of below 0.05 ml/g, particularly less than 0.02 ml/g.
The distribution of pores with a pore diameter of not more than 0.05 xcexcm can be measured by a nitrogen adsorption method or the like. In this range, it is preferred that the pores have an average pore diameter in accordance with the size of a target reaction compound and usually 20 to 200 xc3x85, particularly 30 to 120 xc3x85. The crystal structure of the zirconia portion in the catalyst has an S28.2/S30.2 ratio of not more than 1.0, particularly not more than 0.05. The presence of substantially no monoclinic crystal structure will provide a higher catalytic activity. The solid acid catalyst of the present invention can exhibit an acidity higher than that of 100% sulfuric acid (Hammett acidity function H0 is xe2x88x9211.93).
It is more preferred that the weight of the alumina portion in the total weight of the alumina portion and the zirconia portion in the catalyst is 5 to 90% by weight, preferably 5 to 50% by weight, and more particularly 10 to 40% by weight. Below this range the mechanical strength of the catalyst is decreased and zirconia is difficult to stabilize. Above this range the catalytic activity is lowered relatively. The total weight of the zirconia portion and the alumina portion in the catalyst is preferably not less than 70% by weight, particularly not less than 80% by weight from viewpoints such as catalytic activity and the strength of a shaped material.
The solid acid catalyst of the present invention, if desired, may preferably contain metal components selected from groups 8, 9, or 10 when it is used in conversion reactions such as isomerization. As the metal components to be used for the catalyst of the present invention, selected from groups 8, 9, or 10, there can appropriately be used platinum, palladium, ruthenium, nickel, etc. It is preferred to use them in the form of compounds compared to use of the metal itself. The metal compounds may be used either as anhydrides or as hydrates. Further, the metal compounds may be used singly or mixtures of two or more of them. As for the amount of the metal compounds to be added, it is preferred that they are added such that the total amount of elements of group 8, 9, or 10 in the solid acid catalyst is 0.05 to 10% by weight, particularly 0.1 to 5% by weight.
There is no particular limitation on the method of supporting the metal components. However, an impregnation method such as spraying or dipping, an ion exchange method, etc. can be used advantageously. The above-mentioned supported catalyst is calcined in a gaseous atmosphere such as air or nitrogen at a temperature of 300 to 700xc2x0 C. for 0.1 to 20 hours in order to increase the activity of the catalyst.
The catalyst of the present invention is not powder but in a shaped form so that a catalyst of 0.5 to 20 mm in size can be obtained with ease. Usually, the catalyst having an average particle diameter of 0.2 to 50 mm, particularly 0.5 to 20 mm, is used.
The mechanical strength of the catalyst, obtained as the strength of a side surface crushing strength of a cylindrical pellet of 1.5 mm in diameter, is not less than 2 kg, preferably not less than 3 kg, more preferably 4 to 20 kg. The shaped solid acid catalyst of the present invention maintains its form after it is left in water. Pellets that do not maintain their form in water powder or crack during the step of supporting in the production of a catalyst or during catalytic reactions, which will lead to a decrease in yield or troubles in the process so that such pellets are undesirable in practice.
Acid-Catalyzed Reaction
The acid-catalyzed reaction to which the solid acid catalyst of the present invention is applicable includes those conventional acid-catalyzed reactions in which Lewis acid catalysts, typically aluminum chloride base catalysts or Broensted acid catalysts, typically sulfuric acid, are used. Examples of such reactions include various reactions such as isomerization, disproportionation, nitration, decomposition, alkylation, esterification, acylation, etherification, and polymerization. More specifically, the catalyst of the present invention can be used in an esterification reaction of methacrylic acid, etc., decomposition reaction of cumene hydroperoxide, alkylation reaction of phenol, ring-opening polymerization reaction of tetrahydrofuran, decomposition reaction of flons, oxidative coupling reaction of methane, etc. In particular, it can preferably be used in conversion reactions such as isomerization, decomposition, acylation, etherification, and esterification. Main reaction targets include hydrocarbons, i.e., hydrocarbons and hydrocarbon derivatives such as those derived by attaching substituent groups to the hydrocarbons, particularly hydrocarbons or oxygen-containing hydrocarbon compounds. More particularly, the catalyst of the present invention is preferably used in conversion reactions of hydrocarbons. Examples of the conversion reactions include isomerization, decomposition, acylation, etherification, and alkylation, etc.
The target of isomerization is preferably hydrocarbons in a petroleum fraction having a boiling point in the range of about xe2x88x9220xc2x0 C. to 150xc2x0 C. In particular, the catalyst of the present invention is preferably used in a reaction in which a straight chain paraffin is isomerized into a branched paraffin, or an olefin or an aromatic compound is hydrogenated to form a noncyclic or cyclic paraffin, and then further isomerized. As for the conditions for the isomerization of hydrocarbon compounds, a preferred temperature is in the range of 100 to 300xc2x0 C., particularly 120 to 240xc2x0 C., a preferred pressure is in the range of 1 to 50 kgf/cm2, a preferred LHSV is in the range of 0.2 to 10/hr, and a preferred hydrogen/raw material proportion is in the range of 0.2 to 10 mol/mol.
Treatment in Oxidizing Atmosphere
The catalytic activity of the catalyst of the present invention can be increased by the heat treatment of it in an oxidizing atmosphere before or after use. Usually, the heat treatment is carried out at 300 to 500xc2x0 C. in an atmosphere in which oxygen exists, such as air. The content of oxygen in the atmosphere is preferably 0.1 to 50% by volume, particularly 1 to 30% by weight. Mixtures of nitrogen and oxygen and of nitrogen and air, air, etc. can be used advantageously. In particular, a treating temperature of 350 to 480xc2x0 C. and a treating time of 0.1 to 100 hours are preferred. The treating pressure may be a reduced pressure, atmospheric pressure, and super-atmospheric pressure. Treatment at atmospheric pressure is convenient and preferable. Since the treatment in an oxidizing atmosphere is considered to dry the catalyst and oxidize and remove the material adsorbed thereon and the deposits attached thereto to thereby activate the catalyst, it is preferred that the air to be used contains a reduced amount of impurities such as moisture, particularly before the catalyst is used. More specifically, there can be used preferably a dehumidified atmosphere of which the relative humidity at 20xc2x0 C. has been decreased to not higher than 5xc2x0 C. If the treating temperature is too high, the properties of the catalyst change while too low a treating temperature results in an insufficiently dried catalyst. In either case, the activity of the catalyst decreases. This treatment is effective for a catalyst which has been left to stand in the air for a period of not shorter than 1 day, particularly not shorter than 10 days after heat treatment such as calcination in the course of the preparation of the catalyst or the catalyst used in acid-catalyzed reactions. When the treatment is carried out in a non-oxidizing atmosphere (in an air stream containing no oxygen), also, the activity of the catalyst decreases.
After treatment in an oxidizing atmosphere, the adsorption of moisture to the catalyst must be avoided. For this purpose, it is preferred that the treatment is conducted after the catalyst is introduced into a reaction apparatus or reactor and start the target acid-catalyzed reaction without introduction of the air substantially. When the target acid-catalyzed reaction is carried out in a reducing atmosphere such as a hydrogen atmosphere, it is preferred that the reaction not be started before the atmosphere can be replaced by an inactive atmosphere such as an inert gas, e.g., nitrogen gas or rare gas such as argon. It should be noted that since its activity will not decrease greatly when the catalyst is exposed to the air for a period of about 1 day, in the case of a small scale reaction apparatus, the treatment in an oxidizing atmosphere may be carried out outside the reaction vessel, and then catalyst is introduced into the reaction vessel.
The treatment in the above-mentioned oxidizing atmosphere can be applied to the regeneration of a catalyst which has been used in a reaction apparatus or a reactor and whose activity has decreased. In particular, when a carbonaceous substance, such as xe2x80x9ccokexe2x80x9d, is deposited on the catalyst, it is preferred that the concentration of oxygen is adjusted to 0.1 to 20% by volume, particularly 0.2 to 5% by volume so that the carbonaceous substance is not oxidized abruptly.